Joists are commonly used in the construction industry to span a distance between opposing walls and provide a structural support for a floor, roof or the like. Joists can be comprised of a variety of materials including softwood, wood based laminates, and metal, particularly steel.
Steel joists can be constructed in an open web configuration, which generally consists of spaced apart upper and lower chord members which extend longitudinally thereof and are fastened together by a zig-zag web. Such open web joists are typically manufactured from hot-rolled steel structural members namely the upper and lower chords and the webs. The webs typically can be comprised of hot-rolled steel rods, which are formed into a zigzagged pattern and welded to the upper and lower chords. Integral parts of the web are the end angled supports that connect the ends of the lower chord to the upper chord to counter load stresses at the ends of the joist. Open web joists are normally top chord bearing meaning that they are supported by the underside of the top chord, so that the top chord extends longitudinally beyond the bottom chord and the end angle supports to provide bearing interface with the opposing walls.
Open web joists are by their nature highly customizable in terms of their load bearing capabilities. Both chords and the zig-zag web can be made from different thickness of steel, and the members constituting the zig-zag web can vary in thickness along the length of the joist. The webs are open in the sense that there is a space between the rods longitudinally along the central web section that can receive utilities such as wires, pipe work, air ducts or the like. Open web joists can be concentric, meaning that the load being supported exerts forces that substantially pass through the centres of gravity of the joists. If the joists are loaded otherwise, they are termed eccentric.
The joist industry has introduced various types of composite steel-concrete non-combustible floor and roof systems for the construction industry, in which the top chords are embedded into a concrete slab, such a slab having both load bearing and fire resistant properties. Examples of composite joists can be found in U.S. Pat. Nos. 5,941,035, 4,741,138, 4,454,695 and U.S. Publication No. 2002/0069606 A1. A composite joist design permits the top chord member of a joist to be designed with less steel in comparison with non-composite systems since the concrete slab when properly bonded to the upper steel joist provides additional load support for the floor or roof system.
Generally speaking, for a structural joist member to be composite it must have means to mechanically interlock with the concrete to provide sheer bonding. It is generally difficult and costly to design steel and concrete composite floors using steel joists. Simply affixing vertical studs to the top chord is forbidden by safety regulations in many jurisdictions which state that structural members cannot have objects extending above a structural floor member that will encumber the walking path of a worker.
The methods for providing sheer bonding between the joist and the concrete in a composite joist are generally expensive to produce in the prior art.
Furthermore, camber (defined as a slight arch added to the joist) has been introduced into the open web joist technology to offset the deflection associated with dead loads such that only the live load deflection of the joist needs to be accounted for in designs of the joist. However large machines or jigs are needed to impart the camber to the chords of the joist where typically the web resists the cambering process.
Moreover, hot-rolled open web joists are typically coated or finished with a coloured primer. Steel joists manufacturers typically use large tanks of paint into which completed welded joist assemblies are dipped to receive a coating of primer paint. However, the process has become more expensive due to environmental considerations when using dipped tanks containing volatile solvents.
Furthermore construction with open web joists is dependent on skilled labour which in many instances sets the critical path schedule on many construction projects during busy construction season periods when skilled labour is in highest demand. Because both the manufacture and usage in construction are labour intensive, open web steel joists are costly, so that their use is viable only in larger commercial and industrial structures requiring spans near 40 feet and above.
An alternative approach to the open web steel joist is the cold-formed steel joist. Cold-formed steel structural designs have been used in floor and roof joists in the building construction trade for some time. However prior art cold-formed steel joists have found limited application due to the high costs of construction assembly, and are not cost effective for span lengths much above 24 feet usage is restricted to single and multi-family housing, and to commercial low rise structures.
Provided that light gauge steel is used, cost effective mass manufacture of cold-formed steel joists is practical because highly automated cold forming operations such as roll-forming are commercially available. Joists in the prior art are produced by cold-forming a single piece of sheet metal into a joist comprising a top chord, a web and a bottom chord forming a continuous single structure, and are predominately used in bottom chord bearing conditions. These joists are generally eccentric in that the load forces do not pass through the centre of gravity of the joist. The most common example from prior art is the C-shaped joist which has a cross sectional profile like the letter C. Other examples of cold rolled constructions are shown in U.S. Patent Publication Nos. 2002/0020138 A1 and 2003/0084637 A1.
Composite fire rated floor structures constructed using cold-formed joists are commercially available. Examples are Hambro D510 and Speed Floor both of which have end attachments that are welded, bolted or screwed onto a single strip cold-formed section to provide a top chord bearing joist. However these provide only limited load capacity due to the nature of the localized connection of the end attachments to the cold-formed joist member. Further, they are costly to produce. Cold-formed joist manufacturers provide holes longitudinally along the central web section that are sized to receive utilities for follow-up trades. Since cold-formed joist material can be pre-finished (i.e. the coils of steel can be galvanized or painted) the manufacturing process is less harmful to the worker and environment than the open web coating process described above.
Although cold-formed joists possess superior surface finishes, and can be mass manufactured in a cost-effective manner because there is very little dependency on manpower involved relative to the open web joist technology, current state of the art cold-formed joist technology does not fully exploit the inherent strength to mass ratio of steel, nor does it optimize material usage throughout the length of the joist. The same thickness of steel is used in both of the chords and the web, this thickness being constant along the length of the joist. Eccentric designs have a tendency to be unstable under load due to a mechanical moment about the longitudinal axis. Consequently substantial bracing is required between joists to counteract this effect.
These properties compare unfavourably with the open web steel joist where the chords and the web may be of different thickness and the web member thickness may be varied over the span length in response to loading requirements.
Accordingly, a joist and method of producing said joist that can utilize the beneficial attributes while avoiding the drawbacks from each of the open web joist technology and cold-formed joist technology is desirable. Further, it is desirable to manufacture the joist using automated cold-forming methods as opposed to the labour intensive welding and handling methods employed in open web steel joist construction. It is also desirable to have cost effective fire rated composite floors and roofs based on cold-formed steel joists integrally attached to a concrete slab.
Also both open web and cold-formed steel joist floor and roof structures normally require bridging systems, comprising steel members spanning the gap between joists in a floor or roof assembly, to stabilize the assembly from any lateral movement or rotational movement about the longitudinal direction in response to applied loading. It is common practice to weld bridging in place between open web joists, while cold-formed joist systems have bridging structures that commonly use screws or welding for fastening. Consequently a cost effective means to provide bridging between joists is highly desirable.